In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA ≧0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 laser lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE, Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
Several problems arise when a resist film is exposed in the presence of water. For example, the acid once generated from a photoacid generator and a basic compound added to the resist can be partially leached in water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects.
These drawbacks of the ArF immersion lithography may be overcome by providing a protective coating between the resist film and water to prevent resist components from being leached out and water from penetrating into the resist film (see the 2nd Immersion Workshop, Resist and Cover Material Investigation for Immersion Lithography, 2003).
With respect to the protective coating on the photoresist film, a typical antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCs are made of fluorinated compounds having a low refractive index, such as perfluoroalkyl polyethers and perfluoroalkyl amines. Since these fluorinated compounds are less compatible with organic substances, fluorocarbon solvents are used in coating and stripping of protective coatings, raising environmental and cost issues.
Other resist protective coating materials under investigation include water-soluble or alkali-soluble materials. See, for example, JP-A 6-273926, Japanese Patent No. 2,803,549, and J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 615, 2005. Since the alkali-soluble resist protective coating material is strippable with an alkaline developer, it eliminates a need for an extra stripping unit and offers a great cost saving. From this standpoint, great efforts have been devoted to develop water-insoluble resist protective coating materials, for example, resins having alkali-soluble groups such as fluorinated alcohol, carboxyl or sulfo groups on side chains. See WO 2005/42453, WO 2005/69676, JP-A 2005-264131, JP-A 2006-133716, and JP-A 2006-91798.
As means for preventing resist components from being leached out and water from penetrating into the resist film without a need for a protective coating material, it is proposed in JP-A 2006-48029, JP-A 2006-309245, and JP-A 2007-187887 to add an alkali-soluble, hydrophobic, high-molecular-weight compound as a surfactant to the resist material. This method achieves equivalent effects to the use of protective coating material because the hydrophobic compound is segregated at the resist surface during resist film formation. Additionally, this method is economically advantageous over the use of a protective film because steps of forming and stripping the protective film are unnecessary.
It is believed that independent of whether the alkali-soluble surfactant or the resist protective coating material is used, water droplets remaining on the resist film or protective film after scanning cause failure (or defects) in pattern formation. The ArF immersion lithography systems commercially available at the present are designed such that exposure is carried out by scanning the wafer-mounted stage at a speed of 300 to 550 nm/sec while water is partly held between the projection lens and the wafer. In the event of such high-speed scanning, unless the performance of the resist or protective film is sufficient, water cannot be held between the projection lens and the wafer, and water droplets are left on the surface of the resist film or protective film after scanning. Such residual droplets can cause defects to the pattern.
To eliminate defects owing to residual droplets, it is necessary to improve the flow or mobility of water (hereinafter, water slip) on the relevant coating film and the water repellency of the film. It is reported effective to increase the receding contact angle of the resist or protective film with water. See 2nd International Symposium on Immersion Lithography, 12-15 Sep., 2005, Defectivity data taken with a full-field immersion exposure tool, Nakano et al.
For improving the water repellency of a coating film, introduction of fluorine into a base resin is effective. For improving water slip, combining water-repellent groups of different species to form a microdomain structure is effective. See XXIV FATIPEC Congress Book, Vol. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97 (1997). According to these reports, when a water molecule interacts with methyl and trifluoromethyl groups, it orients via its oxygen and hydrogen atoms, and the orientation distance between water and methyl is longer. Thus a resin having not only water repellent fluorinated units introduced, but also both fluoroalkyl and alkyl groups incorporated is improved in water slip because of a longer orientation distance of water.
One exemplary material known to have excellent water slip and water repellency is a copolymer of α-trifluoromethylacrylate and norbornene derivative (Proc. SPIE, Vol. 4690, p 18, 2002). While this polymer was originally developed as a highly transparent resin for F2 (157 nm) lithography resist materials, it is characterized by a regular arrangement of molecules of water repellent α-trifluoromethylacrylate and norbornene derivative in a ratio of 2:1. This characteristic arrangement increases the orientation distance of water relative to the resin and improves water slip. In fact, when this polymer is used as the base polymer in a protective coating for immersion lithography, water slip is drastically improved, as described in JP-A 2007-140446 or US 20070122736.
Another example of the highly water repellent/water slippery material is a fluorinated ring-closing polymerization polymer having hexafluoroalcohol groups on side chains. This polymer is further improved in water slip by protecting hydroxyl groups on side chains with acid labile groups, as reported in Proc. SPIE. Vol. 6519, p 651905 (2007).
A material having good water slip performance is required not only from the standpoint of defects, but also from the standpoint of productivity. The immersion lithography needs higher throughputs than ever. For improved productivity, the exposure time must be reduced, which in turn requires high-speed scanning operation of the stage. In order to move the stage at a high speed while holding water beneath the lens, it is desired to have a resist material or resist protective film having higher water slip performance.
The highly water repellent/water slippery materials discussed above are expected to be applied not only to the ArF immersion lithography, but also to the resist material for mask blanks. Resist materials for mask blanks suffer from problems including a change of sensitivity during long-term exposure in vacuum and long-term stability after coating. With respect to the control of sensitivity changes in vacuum, an improvement is made by a combination of acid labile groups of acetal and tertiary ester types (U.S. Pat. No. 6,869,744). It is believed that after coating of a resist material, an amine component is adsorbed to the resist film surface whereby the resist varies its sensitivity or profile. A method of modifying the surface of a resist film for preventing adsorption of an amine component to the resist film has been devised.